The disclosed embodiments concern a method for limiting the steering angle of an aircraft control surface under certain flight conditions, especially when the aircraft is caused to yaw and the steering control surface is operated at maximum clearance. The disclosed embodiments also concern a system for implementing said method.
The disclosed embodiments find application in the field of aeronautics and, in particular, in the field of an aircraft's control surface drive.
In an aircraft, the rudder is a mobile flap mounted on the tail fin of the aircraft and operated from the cockpit to change the direction of the aircraft. The tail fin constitutes a relatively important surface on the aircraft the essential role of which is to assure stability in the route of the aircraft. The tail fin is able to withstand forces which may be relatively significant. However, these forces must not surpass a certain level because this would involve breakage of the tail fin. These forces are dependent upon the flying conditions of the aircraft and, in particular, the speed of the aircraft. Also, to limit these forces on the tail fin, there is a system, which is installed on the majority of the airplanes, which makes it possible to limit control surface deflection under certain flying conditions, i.e. to limit the clearance allowed by the control surface. This limitation is obtained through stops located on both sides of the control surface and the position of which is controlled through struts. Limiting the angle of deflection of the steering angle of the control surface is directly connected to the speed of the aircraft. Thus, the faster the aircraft goes the more the clearance in the control surface is reduced; consequently the closer the stops are to the control surface. On the other hand, the slower the aircraft goes the higher the steering angle allowed; consequently more stops are farther away from the control surface. Under normal aircraft flight conditions, the rudder is used at landing, to align the aircraft with the landing strip, and when the aircraft is taxiing. In these two cases, the aircraft is at a low speed. The steering angle allowed by the control surface can thus be elevated.
Under abnormal aircraft flight conditions, for example, when there is engine failure, the rudder may be used to compensate for dissymmetry that occurs at the moment the motor loses output. In fact, when a motor ceases to function, the aircraft is thrown into yaw and flies askew, i.e., the aircraft is no longer in the flight line. It is then necessary to activate the rudder to bring the aircraft back into the flight line. Under these conditions, it is important that the clearance allowed by the control surface be sufficiently elevated to allow righting of the aircraft.
The traditional system for limiting the steering angle of the control surface is set up so that the pilot may compensate for the effects of such an engine failure. In other words, the traditional limitation is calculated so as to allow the pilot sufficient control to be able to compensate for dissymmetry generated by engine failure.
However, this traditional system does not take into account other abnormal situations which may require implementation of the control surface deflection.
In fact, nothing prevents the pilot from sending several successive commands for control surface deflection, in opposite directions, with angles reaching the allowed maximum clearance. For example, if the pilot first orders a control surface deflection in one direction, for one reason, then a second control surface deflection in the opposite direction, for another reason, then a third control surface deflection in the first direction, at the maximum steering angle, then the forces which affect the tail fin can become so significant that it will cause the structure of the aircraft to be shaken.
In another example of abnormal flight conditions, if an aircraft is thrown into yaw, following a control surface deflection order or when there is a motor failure, the aircraft will be flying askew. The wind will then be hitting it sideways. If, at that moment, the pilot orders a control surface deflection at the maximum angle, to regain the flight line, then the rudder will find itself traveling straight into the wind. Forces will begin to weigh heavily on the control surface. If the pilot orders a new control surface deflection, in the opposite direction, at a maximum angle, then the forces hitting the tail fin may exceed the force the aircraft was designed to bear.
The forces endured by the tail fin may then reach and even surpass the limits imposed by the construction of the aircraft itself. In the worst cases, the tail fin may break under the effect of these forces, or constraints, and could cause the aircraft to crash.